The present invention relates to automatic microphone control systems and, more particularly, to an enhancement of the invention disclosed in U.S. Pat. Nos. 4,489,442, issued to Carl R. Anderson, et al. entitled "Sound Actuated Microphone System" and 4,658,425, issued to Stephen D. Julstrom, entitled "Microphone Actuation Control System Suitable for Teleconference Systems." U.S. Pat. Nos. 4,489,442 and 4,658,425 are both owned by the same entity as the present application.
The contents of U.S. Pat. Nos. 4,489,442 and 4,658,425 are incorporated herein by reference, as if fully set forth below. For ease of reference, U.S. Pat. Nos. 4,489,442 is hereinafter referred to simply as the "Anderson patent"; 4,658,425 is hereinafter referred to as "the Julstrom patent".
It is a common practice in audio engineering to use multiple microphones placed at different locations throughout rooms such as conference rooms, classrooms, or on a stage wherein multiple talkers voices need to be either amplified and/or recorded. In such a system, the outputs of the microphones are usually added (combined) in an audio mixer, the output of which might feed into an amplifier, a recording device, or a transmission link to a remote location.
Multiple microphones are used to insure that each person's voice can be picked up by at least one microphone at a relatively close distance to his mouth thereby helping to insure that the audio quality, including intelligibility, is sufficient for each person. In a conference room, classroom, or on a stage, using only one microphone invariably means that some talkers will be farther away from the microphone than others. The talkers who are far from the microphone might not have their voices heard well above the rooms background noise. Using multiple microphones results in a higher ratio of direct sound from the talker's voice to room noise and reverberation at each microphone. However, the use of multiple microphones that all pick up the unwanted ambient noise and reverberation as well as the desired talker's voice creates several other problems.
The Anderson patent teaches a method and apparatus for determining if a given microphone should be turned ON or OFF by using two, back-to-back cardioid microphone elements. If a talker's voice originates from in front of the microphone, then the signal heard by the front-oriented microphone will be louder than the rear-oriented microphone, and the microphone should then be turned ON.
The output signal from a cardioid microphone element can be plotted in polar coordinates which will produce the heart-shaped graph shown in FIG. 3 of the Anderson patent. A sound wave incident upon a cardioid microphone element at an angle theta, will have an output level represented by the vector "S". FIG. 3 is a polar coordinate plot of the cardioid element as a function of the angle of incidence of an acoustic wave. A wave that impinges upon the element at 0 degrees will produce the highest possible output; a wave that impinges upon the rear of the element, i.e. at 180 degrees, in theory, produces no output. The combination of the polar responses of the elements with the circuitry described in the Anderson patent yields a direction-sensitive microphone which will turn ON if a sound originates within a predetermined angle in front of the microphone; it is spatially selective.
While the invention disclosed in the Anderson patent is effective in providing spatial selection of microphones, such spatial selection is often insufficient to avoid unwanted detection of an audio source. When several microphones are placed side-by-side, the spatial selectivity of the microphones is inadequate to avoid turning ON several of the microphones if a sound source originates within the sound-sensitive space of more than one of the microphones.
In applications where multiple microphones are required to be able to hear different talkers, it would be desirable to be able to ignore microphones that do not best "hear" the talker's voice.
While the Julstrom patent disclosed a circuit for comparing the outputs of several microphones in an audio sound system and for turning ON only one microphone per talker, the Julstrom patent does not provide any means for spatial selection of microphones; a talker can turn ON a microphone if he is not in front of it.
Accordingly, an audio system that discriminates both on the number of ON microphones per talker and the location or orientation of the source would be an improvement over the prior art.
An object of the present invention is to provide an audio system that identifies if a talker is within some predetermined location with respect to the microphone and identifies the microphone that best hears the talker.